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From: Peter
Newsgroups: rec.aviation.misc,rec.aviation.owning,sci.electronics.design
Subject: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 14:44:34 +0100
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The normal procedure in light airfraft startup/shutdown is to have all
possible avionics disconnected when the engine is being started or
stopped.
Moreover, one is normally advised to have the alternator field circuit
broken at these times (there is a switch for this purpose).
There are many stories going around of somebody's entire avionics kit
getting blown up if this procedure is not followed.
The alternator etc parts are more or less standard automotive parts.
One could get into an interminable discussion as to why these issues
were addressed in motor vehicles decades ago but still persist in
brand new aircraft, perhaps loaded with avionics costing US$100k+, but
I am mainly interested in finding out *exactly* what happens.
I have not found anybody who is able to explain clearly what happens.
The general circuit, certainly in the planes I have knowledge of, is
this:
http://www.peter2000.co.uk/tmp/alternator.jpg
I cannot see any way to generate an overvoltage surge in the +24V
(+28V when the engine is running normally) output as a result of
switching the engine OFF. At this time, as the engine RPM drops, the
voltage regulator will pass an increasing current, until it runs out
of steam. This should be a clean process - unless the regulator design
is defective and it somehow passes a massive field current at the last
moment.
However there appears to be more scope for an overvoltage surge when
switching the engine ON (if the field circuit was operating). This is
because the voltage regulator will be sitting there and passing its
maximum current, for some seconds or more, and when the engine starts
up, and the alternator spins up, the voltage regulator may not reduce
the current fast enough.
There may be more obscure means of generating a NEGATIVE-going surge
on the +24V rail but I can't really see how.
I have also noted that on the aircraft I currently fly, the alternator
is set up so that even with max field current, the normal regulated
voltage (+28V) is not reached until one is up to 1200RPM, which is at
or above what the engine immediately spins up to when correctly
started. While this is an irritation (the battery is not being charged
during taxiing around) it may have been done to minimise the risk of
damage if the engine is started with the field connected.
Can anyone offer any clearly reasoned comments on this scenario?
Peter.
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